Across‐Arc Diversity in Rhyolites From an Intra‐oceanic Arc: Evidence From IODP Site U1437, Izu‐Bonin Rear Arc, and Surrounding Area

Abstract

Felsic magmas from the Izu‐Bonin rear arc have compositions that resemble average continental crust in some respects. In order to understand their origin, we studied 1.1–4.4 Ma tephras in a rear‐arc drill core from International Ocean Discovery Program Expedition 350, Site U1437. They provide a well‐dated record of changing magmatic compositions during the early stages of the most recent episode of Izu‐Bonin arc rifting. Based on our comprehensive recontextualization of published analyses of <7 Ma regional dredged rocks across the arc, basalts to rhyolites are shown to vary in coherent chronological and spatial trends and can be classified into three series: light rare earth element‐depleted volcanic front series; a rift‐related series with nearly flat rare earth element patterns; and light rare earth element‐enriched rear‐arc seamount chain‐type (RASC‐type) series, which are abundant in the studied Site U1437 tephra record. All three series erupted simultaneously between 4.4 and 1.1 Ma, including the RASC‐type rhyolites which erupted until 1.1 Ma in significant quantities. Remarkably, trace element and radiogenic isotope ratios are similar between rhyolites and basalts from the same region. This recontextualization of rhyolite affinity represents a significant departure from existing frameworks. Geochemical modeling shows that fractional crystallization can largely explain <4.4 Ma RASC‐type rhyolites with some additional open system processing evident in rhyolites with >73% SiO2. However, trace element and Hf isotope ratios preclude rear‐arc rhyolite derivation by partial melting of the Oligocene‐Eocene arc basement. Thus, we favor a model where fractional crystallization is more important than crustal melting in producing intra‐oceanic arc rhyolites in this region.

Tuffaceous Mud is a Volumetrically Important Volcaniclastic Facies of Submarine Arc Volcanism and Record of Climate Change

“…The age and composition of tuff‐rich Lithologic Unit IV are similar to those of lava dredged from the Manji summit where mineralized plutonic rocks (tonalites) also are exposed (Ishizuka et al, ; Tamura et al, 2015). The age and composition of tuffs in Lithologic Unit II are somewhat similar to those of rhyolite pumice from Meireki, the next seamount east of Manji (Heywood et al, ; Hochstaedter et al, ; Ishizuka et al, ) and identified in Figure b.…”

“…The chemical evolution of the Izu volcanic front is well known from studies of bulk discrete ash and tuff intervals (sometimes called turbidites), and their individual vitriclasts, in ODP cores from the forearc (Bryant et al, ; Gill et al, ; Straub et al, ). One rationale for IODP Expedition 350 was to do the same for the rear arc, and some studies of discrete ash and tuff intervals from all Lithologic Units except III are underway (Heywood et al, ; Miyazaki et al, ; Sato et al, ; Schindlbeck et al, ). The tuffaceous mud(stone)s are compromised for this purpose because they contain variable amounts of foreign silicate material, bioclasts, and volcanic material from the volcanic front.…”

“…that may reflect long term episodicity in the amount or nature of the local volcanic input. Throughout that depth range, all three parameters vary from values as low as in Unit VII, BAK, and the Quaternary volcanic front, to as high as in nearby RASC lavas and some discrete tuffs in Units II to V (Heywood et al, ; Hochstaedter et al, ; Miyazaki et al, ). Four of our six samples from these units have 143 Nd/ 144 Nd and Nb/Yb ratios that mix toward RASC lavas to which, therefore, they seem related (Figure ).…”

“…As usual, more attention has been paid to the coarser, more obviously volcaniclastic intervals (e.g., Heywood et al, ; Miyazaki et al, ; Sato et al, ; Schindlbeck et al, ). Discrete layers are mostly ash or tuff (i.e., average grain size <2 mm); coarser material (lapilli‐ash, lapilli‐tuff, lapillistone, or breccia) is minor (<5%).…”

The First 10 Million Years of Rear‐Arc Magmas Following Backarc Basin Formation Behind the Izu Arc

“…The Izu arc is a very suitable place in which to study oceanic arcs for many reasons: (1) the weak influence of crustal contamination relative to mature island arcs (Savov et al, 2006;Stern et al, 2003;Tollstrup et al, 2010); (2) many samples are available from across the arc including the forearc, active magmatic arc, rear-arc rifts and cross chains, and backarc basin (Shikoku Basin); (3) the composition of the subduction inputs (the mantle wedge, subducting sediment, and altered oceanic crust [AOC]) is well known (Chauvel et al, 2009;Hauff et al, 2003;Kelley et al, 2003;Plank et al, 2007); and (4) no slab material has been removed into an accretionary complex, resulting in complete subduction. Many geochemical studies have been conducted in this arc (e.g., Heywood et al, 2020, and references therein), and numerous sites have been drilled in the arc front and forearc regions by ODP Legs 125 and 126 and IODP Leg 352, but there had been no drilling in the rear-arc region prior to IODP Exp. 350 (Tamura et al, 2015).…”